How do I import .c4d files on Blender?
Maxon's Cinema 4D is a closed and proprietary tool and there is no way to access the .c4d file in a legal, safe and usable way other than C4D itself.

But you can use one of the many other export methods available.

".obj" , ".fbx" , ".dae" .... for simple mesh transfer and Alembic (.abc) for dynamic and advanced export.